1. Field of the Invention
The present invention relates to a liquid crystal display and a liquid crystal television using a Twisted Nematic (TN) liquid crystal panel to display images.
2. Description of the Related Art
Liquid crystal displays are devices for displaying images using liquid crystal panels adapted to change the light transmission in accordance with voltages applied thereto. As types of the liquid crystal panels, there exist a Twisted Nematic (TN) type, a Vertical Alignment (VA) type, an In-Plane Switching (IPS) type, and an Optically Compensated Bend (OCB) type. Among these types, the TN liquid crystal panels are low in price, and in widespread use.
The TN liquid crystal panel has a structure having nematic liquid crystal with positive dielectric anisotropy intervening between two glass substrates having polarization plates (oriented films) with polarization directions perpendicular to each other formed on the respective surfaces thereof. In the TN type, the axis of the liquid crystal molecule rotates up to 90 degrees between when a voltage is applied and when the voltage is not applied. Further, the light is transmitted when the voltage is not applied, and the transmission of the light is gradually reduced as the voltage applied thereto increases.
Further, the liquid crystal element can be driven with a low voltage, has low power consumption, and has a high contrast property with a wide contrast range.
On the other hand, the TN liquid crystal panels are apt to cause variations or oblique lines on a screen. The TN liquid crystal panels have a rubbing process in forming the pixels. In the rubbing process, grooves are provided to the oriented film transferred on the glass substrate so that the arrangement directions of the liquid crystal molecules become homogenized. Specifically, in the rubbing process, a roller with rubbing cloth wound around the roller is rotated while pressed against the oriented film. On this occasion, if amount and direction of the pressure with which the rubbing cloth is pressed against the oriented film are not even, the grooves cannot uniformly be formed. If the grooves are not uniform, the arrangement of the liquid crystal molecules becomes uneven. Therefore, since there is caused the liquid crystal, which does not have a uniform arrangement after the arrangement of the liquid crystal molecules is changed even if the voltage is applied to reduce the transmission, such liquid crystal causes leakage light, and the leakage light generates the variations and oblique lines on the screen.
In Patent Document 1 (JP-A-2006-323073), in order for enhancing feel of contrast of an image to improve the quality of the image, the luminance of a backlight is controlled in accordance with a state of an input video signal. According to this document, the luminance of the backlight is controlled in accordance with the luminance or the grayscale level of the input video signal.
Further, in Patent Document 2 (JP-A-2006-243591), control is performed when suppressing temperature rise in the inside of a liquid crystal display so that the luminance in the image does not drop beyond a predetermined value.
In Patent Document 3 (JP-A-11-194736), a luminance control range of a backlight is enlarged without complicating the configuration of an inverter circuit.
Further, in Patent Document 4 (JP-A-2007-155819), reduction of the luminance control range is suppressed in a liquid crystal display performing black-insertion driving with a liquid crystal panel of an OCB operation mode. Here, the black-insertion driving is a technology for inserting an image for forming a black display period in a frame separately from the video signal.
Further, in Patent Document 5 (JP-A-2007-150967), when controlling the luminance of the backlight using PWM control, a duty ratio is controlled to extend the life of the backlight.
No technologies disclosed in the Patent Document 1 though 5 are capable of eliminating the variations or the oblique lines on the screen which are inherent in the TN liquid crystal panels. In the related art, the variations and the oblique lines on the screen described above are regarded as being unavoidable problems, and are not improved.